By Amy Norton, Medical Tribune News Service
Combining the various prenatal tests for Down syndrome into one
screening process may better predict which children will be born
with the genetic disorder, according to British researchers.
Currently, doctors use several different tests to gauge a
woman's risk of having a baby with Down syndrome. Some tests are
given during the first trimester of pregnancy, and others in the
second trimester. Whether and when to screen are decisions left up
to the woman and her obstetrician. A woman's risk of having a
Down-syndrome baby increases as she get older, from about 1 in
2,000 at age 20, to about 1 in 100 at age 40.
Researchers at the Royal London School of Medicine claim that
integrating the different tests into one would help identify more
Down-syndrome cases, while reducing false alarms.
Down syndrome arises from a chromosome abnormality. Normally,
everyone inherits 23 chromosomes from each parent to make 23 pairs.
In Down syndrome, there is a an extra copy of chromosome 21; this
abnormality causes mental retardation, as well as serious physical
problems including defects of the heart, stomach and intestines.
In this Thursday's issue of The New England Journal of Medicine,
Dr. N.J. Wald and colleagues reported that integrated prenatal
testing could detect Down syndrome 85 percent of the time, with a
false-positive rate of just 0.9 percent.
This, they contended, would cut the number of invasive
screenings performed on women by four-fifths. Invasive tests, such
as amniocentesis, are used if a fetus is considered at increased
risk for Down syndrome. However, amniocentesis, in which a large
needle is inserted in the woman's abdomen to remove discarded fetal
cells in the amniotic fluid, results in miscarriage in about 1
percent of cases.
Integrated testing for Down syndrome is just a concept right
now. The British researchers arrived at their conclusions by
reviewing studies on the reliability of blood tests, ultrasound and
other measures currently used to detect the birth disorder. They
found that the three tests commonly used in combination during the
second trimester yielded a detection rate of 69 percent and a
false-positive rate of 5 percent overall.
Wald's team theorized that combining these tests with standard
first-trimester tests, such as ultrasound to look at the thickness
of the fetus' neck, would improve detection. They estimated that in
the United States, using the integrated test would detect about 800
more cases of Down syndrome each year, while saving about 1,400
unaffected fetuses from miscarriage due to invasive procedures.
But if integrated testing does come to fruition, it won't catch
on in the United States, according to one expert. Combining first-
and second-trimester tests presents ethical problems, said Dr.
Joshua A. Copel, a professor of obstetrics and gynecology at Yale
University School of Medicine in New Haven, Conn.
Typically, a physician will first look at the neck thickness of
a fetus and perform two types of blood tests between weeks 10 and
14 of pregnancy, according to Copel. The results are available
within a few days.
But the integrated approach would require physicians to withhold
these early results until further tests were performed. ``Three,
four or five weeks would pass until [parents] got the results,''
Copel said.
Deciding how to deal with a Down-syndrome pregnancy is intensely
personal, he and his colleague at Yale, Dr. Ray Bahado-Singh,
pointed out in an editorial accompanying the report. Many parents
want test results as soon as possible in case they decide to end
the pregnancy.
While the new report shows that integrated testing may more
reliably detect Down syndrome, Copel stressed that all of the
prenatal tests for the disorder only measure a woman's risk. They
do not definitively diagnose Down syndrome.
New blood tests and urine-sample tests are currently under
investigation, said Copel. He predicted that these tools will
further improve Down-syndrome detection, without the disadvantage
of making parents wait for the results.
